In one aspect this invention relates to novel, improved techniques for densifying and improving the surface finish of fine grained, isotropic graphites.
The invention also relates, in a second aspect, to novel, improved, densified, carbonaceous artifacts which have a superior surface finish and are obtained by impregnating a fine grained, isotropic graphite with a liquid impregnant and subsequently first polymerizing and then pyrolyzing or carbonizing the impregnant.
And, in two other aspects, this invention relates to novel, improved liquid impregnants for use in the techniques of treating fine grained, isotropic graphites identified above and to the novel artifacts impregnated therewith.
Fine grained, molded graphite parts have residual porosity which detracts from performance in some applications in which they are used, such as in crucibles and the like. For example, this porosity generally admits oxidizing media and allows the gaseous products of oxidation to leave the body, thereby generally lowering its oxidation resistance.
Also, in the case of certain crucibles for melting liquid metals, the surface porosity allows some penetration of the liquid into the crucible and thereby contributes to degradation of the crucible.
Also, graphite parts in contact with glass at high temperature are degraded by glass entering the surface pores of the crucible. This causes sticking and subsequent breaking away of the crucible surface when the contact is broken. The pores also contribute to a general loss of strength.
In order to overcome these difficulties a wide variety of impregnation process have been practiced in the graphite industry.
For example, decomposition of a gas phase to close interconnected pores and prevent unwanted penetration of graphitic material is suggested in U.S. Pat. No. 3,084,394 to Bickerdike.
Impregnation of carbon and other porous bodies with furfural alcohol to fill their pores is described by I. S. Goldstein and W. A. Dreker in Industrial Engineering Chemistry, 52, 57 (1960).
In U.S. Pat. No. 3,628,984, Ishikawa and Teranishi disclose the use of a solution of furfural and acetone with two catalytic additives to impregnate graphite bodies in order to reduce porosity and improve properties obtained after the body is cured in an acid bath and subsequently carbonized. Unfortunately, both catalysts and the acetone are required to achieve the desired results.
A group of liquids that can be cast into pyrolyzable, free-standing bodies, with highly controlled systems of porosity, is disclosed in U.S. Pat. No. 3,859,421. It has recently and unexpectedly been discovered that certain of these liquids can also be used to impregnate fine-grained, isotropic graphites. The subsequent polymerization and pyrolyzation of those liquid impregnants reduces the susceptibility of the impregnated graphitic body to degrative attack by increasing its density and decreasing the average size of the number of interconnected pores opening onto the surface of the body.
That fine grained, isotropic graphites can be successfully treated by the technique just described is important because technical demands for higher strength and quality, etc. and the elimination of anisotropic characteristics have led to the increasing use of such isotropic graphites.
Fine grained graphites, such as those manufactured and sold by Poco Graphite Company, have grain sizes of about 0.001 inch and residual, interconnected pores typically about 0.4 micron in size. These fine-grained, small pore graphites yield a superior surface finish when they are machined.
However, it has been found that even these fine-pored graphites can be impregnated, and surface finish and product performance further improved, using the novel impregnating techniques and impregnants disclosed herein.